The manufacture of seamless steel tubes when involving the use of a rotary rolling mill, is well known.
From the casting of molten steel to the production of the ultimate product, all details involved are described by "The Making, Shaping and Treating of Steel", 7th Edition, published and copyrighted 1957 by the U.S. Steel Corporation, particularly by Chapters 20, 25 and 41 of this text. In its entirety, this text is hereby incorporated as a part of the present specification.
Conventionally a huge investment in equipment and labor is required to provide a rotary rolling mill with a hollow or tubular billet for the mill to roll into a tube of specified wall thickness and length. An ingot must be cast and its top extensively cropped to remove pipe and segregations; the ingot must be reheated and rolled into a billet which must be reheated and pierced by a piercing mill. These have been minimum requirements to obtain a billet suitable for rotary rolling. The billet produced has a thick wall which in the rotary rolling mill is thinned and normally expanded to a larger diameter.
Great economy could be obtained if molten metal could be cast directly into a hollow billet and rolled into the tubular shape desired by the rotary rolling mill, providing a sound product could be obtained and labor cost could be kept down. However, any casting involves the problem of the formation of pipe and segregations in the upper portion of the casting, the molds that would have to be used must be small as compared to a conventional ingot mold, introducing handling problems and, therefore, a high labor cost, and a casting as such is ordinarily unsuitable for processing by a rotary rolling mill. These and other factors have presented a problem which has never been solved by the industry.